In the field of liquid-based solid imaging, alternatively known as stereolithography, compositions have been developed which are capable of generating solid objects having the properties of epoxies and/or acrylates. Solid imaging generated objects made from previous epoxy and/or acrylate compositions provide a prototypical representation of the physical shape of plastic articles made on a production basis out of materials such as ABS, nylon, polyethylene, polypropylene, etc. However, such compositions lack the material properties that give users of the prototypes a sense of look and feel for the object when produced in the production material. Such a lack of look and feel accuracy in product prototyping is not just an aesthetic issue. The look and feel of a prototype also has significant engineering, design, packaging, labeling, and advertising implications.
For example, squeeze bottles, such as those used for dispensing dish soap, are designed to be attractive in shape, easy to grasp, and easy to squeeze. Typically such bottles are made from polyethylene type polymers. Previous epoxy and/or acrylate compositions used in solid imaging were capable of producing articles that have the same attractive shape. However, the stiffness of the prototyped articles made from these materials was likely to mislead the designer and evaluator of the article relative to such issues as, for example, wall thickness and surface radius design. For example, a commercial solid imaging resin, Somos.RTM. 2100 (E. I. DuPont De Nemours, Inc., Wilmington, Del.), produces articles having lower stiffness than polyethylene. Bottles made from this material do not provide adequate resistance to squeezing such that enough friction occurs between a person's fingers and the bottle. A person holding a Somos.RTM. 2100 prototype bottle of liquid soap is likely to squeeze too hard in order to generate enough friction to keep the bottle from slipping. As a consequence, the soap is likely to be dispensed prematurely. The designer of the bottle might then be misled into making the wall thickness of the bottle greater in order to improve its stiffness. But such a design change would lead to-a bottle that is too stiff when manufactured with polyethylene. Similar problems are generated when other much stiffer epoxy and/or acrylate compositions are used to prototype articles such as bottles. A designer might be led to decrease the wall thickness of the bottle due to the stiffness. Or for example, since sharper radii may not feel as comfortable during squeezing when stiffer materials are used, the designer may be misled into re-designing the bottle with greater radii. This may affect the bottle squeezability when manufactured in polyethylene or may reduce the aesthetic appeal of the bottle shape.
Other examples may be made regarding the importance of appearance of an article when made out of certain materials. For example, use of a transparent prototype composition or an overly opaque composition may mislead those viewing the article into incorrect assumptions regarding appropriate packaging, labeling, coloring, and advertising of a product.
Other considerations when trying to utilize solid imaging for prototyping include photospeed, resistance to humidity, low potential for hydrolysis, similar coefficient of friction, dimensional accuracy, ability to span without supports during fabrication, and wide process latitude.
Japanese Patent Application Hei 2-75618 describes epoxy and acrylate compositions for use in optical molding. The compositions contain at least 40 wt % of alicyclic epoxy resin with at least two epoxy groups in each molecule.
U.S. Pat. No. 5,476,784 describes cationic epoxy and acrylate compositions for use in solid imaging. The compositions may comprise from 5-40% by weight of at least one OH-terminated polyether, polyester or polyurethane. In the examples given, the polyether polyols formulations provide lower elongation at break properties that the elongation at yield properties of most low-density polyethylenes. Additionally, the patent teaches that the epoxy content is to be from 40 to 80% of the formulation by weight. Compositions made with this epoxy content are likely to produce cured articles having a higher modulus than that of polyethylene.